1. Field of the Invention
The present invention relates to a method and an apparatus for determining a frequency for the sampling of an analog image, and, here, in particular, to a method for determining a frequency for the sampling of an analog signal provided to a digital screen, so as to display an image on the digital screen. Further, in particular, the present invention relates to an apparatus for generating digital data from analog image data, so as to display an image based on the generated image data on a digital screen.
2. Description of the Related Art
Conventional computers and/or calculating units include elements, eg graphic cards, so as to provide graphic information generated in the computer, such as eg images, for display on an external device, such as a screen. Based on the digital signals, which are provided by the computer and/or its central processing unit (CPU), the conventionally used graphic cards generate corresponding image data suitable for controlling a screen. In many applications the display device associated with the computer includes the screen, an analog screen, which comprises a cathode ray tube. In order to be able to provide the required data for this case of application, which has been exclusively existing up until a few years ago, the graphic card includes a digital/analog converter, so as to convert the image data generated by the graphic card into an analog signal, eg a RGB signal, which then enables the controlling of the screen. In addition to the analog image data signals (RGB signals), the horizontal and vertical synchronization signals are also output to the screen, which are required for a proper rendition of the image data on the screen.
Recently, however, so-called digital screens have been increasingly used, eg LCD-screens or LCD-monitors (LCD=liquid crystal display), which, in contrast to screens with cathode ray tubes, require digital control. In this case it is necessary to subject an analog video signal applied to a video output of a computer/calculating unit to digital further processing in the screen/monitor. At first, this makes it necessary to digitize the analog video signal once more with a sampling frequency. In order to reconstruct the output data with an as exact a sampling frequency as possible it is therefore desirable to sample the analog signal with the original frequency and with a correct phase position, that is with the frequency and phase position by which the analog video signals were generated from the digital data in the graphic card at the output of the computer. The phase position refers to the displacement of the sample signal relative to the generated sample signal, with the phase position generally being indicated in degrees, eg 0 degrees, which corresponds to no displacement, or 180 degrees, corresponding to a displacement by a half clock period.
FIG. 1 schematically represents the waveform of an analog video signal (see FIG. 1A) at the output of a digital screen. Also represented in FIG. 1B is a sample clock being ideal for the sampling of this applied analog signal. T refers to a period of the sample clock.
While the generation of images on analog screens using the analog video signals generated by the graphic card is generally problem-free and, in particular does not result in any visible artifacts, the repeated sampling of an analog signal based on an original digital signal does represent a problem, since artifacts in the represented image may arise on the basis of the repeated sampling in the digital screen, with these artifacts being visible to the viewer. In order to avoid such artifacts, various approaches are known in the state of the art, which will be set forth briefly below.
For example, in the U.S. Pat. No. 6,268,848, a method is described, by means of which visible errors in an image being displayed on a digital monitor may be avoided in that an automatic sample control system is employed, in which, for sucessive image frames, the image content of which remains essentially the same, a phase of the sample clock, for a repeated sampling of the received analog signal, will be changed until a maximum sample value is reached. The phase value achieved with the maximum sample value will then represent the phase-shift of the sample clock which is optimum for the sampling of this frame.
The U.S. Pat. No. 6,147,668 describes a digital display unit, by means of which display artifacts, which are generated on the basis of the aliasing effects of high-frequent interferences in analog display signals, are avoided and/or minimized. Similar to the U.S. Pat. No. 6,268,848, a modulation is also carried out, so as to provide the sample clock signal with different phase-shifts for successive lines or frames so that, on the basis of this modulation, the analog display signal is sampled for a display on the digital display element at different sample points for the same pixel in different frames.
As may be seen, in the above-described approaches only one sample phase is varied, whereas the sampling frequency remains unchanged. The approaches described in the two US patents above use sample clocks, which are derived based on the horizontal and vertical synchronization signals provided together with the analog video signal. The synchronization signals represent the reference signals for the digital screen, with which a clock generator in the screen and/or in the screen control is locked, so as to generate a suitable sample clock based on the reference signal.
Conventionally, the generation of the reference signal for the clock generator is effected such that, based on the received synchronization signals of the analog signal, access is made to a look-up table, from which a reference value suitable/ideal for this synchronization signals is selected, which will then be provided to the clock generator as a reference clock and/or reference frequency for generating the sample clock.
The above approaches will only function if it is ensured that the synchronization signals and/or the reference signal, which is associated with the analog signal, actually renders the frequency of the digital signal, on the basis of which the analog signal has been generated. In this case the sample clock generated by the clock generator in the digital screen and/or in the control of the same matches this frequency. This marginal condition, however, does not apply for all graphic cards and, as a rule, is only fulfilled for very highly advanced graphic cards. Other graphic cards, eg less expensive graphic cards, comprise tolerances resulting in that the frequency used in the graphic card comprises deviations to the frequency which is signalized to the digital screen as an optimum/ideal sampling frequency. Conventionally, these deviations are in the range from 1% to 5% of the sampling frequency signalized to the screen.
In such cases, the above-described approaches for sampling analog signals in digital screens for avoiding artifacts or interferences in the display of the image are only employable under certain conditions, since, here, a frequency error is present when sampling the analog signal, which requires further correction.